This invention relates generally to blending an airfoil surface to a platform surface and more particularly to generating a smooth blending fillet surface between an airfoil shape and a restricted platform surface upon which the airfoil is mounted.
Typically, a turbine blade designer uses a computer-aided design (CAD) system or a computer-aided manufacture (CAM) system to generate a fillet surface for blending an airfoil surface to a platform surface. The process of creating a fillet blend on a CAD system requires identifying the adjacent non-tangent faces of a computational solid model of a bucket blade, then executing a system function that adds a tangent blend between the adjacent faces. The subject faces are that of the vane surface at its root (or base) and the dovetail platform to which it is attached.
One problem with using either the CAD system or the CAM system to generate the fillet surface for blending the airfoil surface to the platform is that the solution provided by these systems typically assumes a single fixed radius blend and an unrestricted platform size. Assuming a single fixed radius blend and an unrestricted platform size results in a fillet surface that is undesirable. In the application of long bucket blades it is undesirable to have a fillet blend of constant (single fixed) radius. Compound (multiple radii) blends are required in order to separate centrifugal and tangential vane stresses at the root. Also, assuming an unrestricted platform size results in a fillet surface that is truncated and disrupts smooth steam flow at the root of the steam path. Since the fillet surface is undesirable, a skilled artisan has to usually finish the surface to meet the design requirements. Associated with adding a compound (multiple radii) blend is the preparation of the vane surface. The vane surface must be largely free of inflections, voids and reversing curvatures that will interfere or perhaps prevent the computation of a tool path by subsequent CAM software. CAD systems require a labor intensive process of analysis and editing, to identify and remedy the aforementioned surface anomalies. Therefore, there is a need for a system and method that can generate a fillet surface for blending the airfoil surface to a restricted platform with multiple varying radii.